Ch 23 notes - Chemistry 140C Spring 2009 (K. Albizati)...

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Chemistry 140C Spring 2009 (K. Albizati) 1 Chapter 23 Ester Enolates and the Claisen Condensation -dicarbonyl compounds are versatile compounds which have significant industrial importance. They are also an important class of biochemical intermediates. -Ketoesters via the Claisen Condensation General Form Example
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Chemistry 140C Spring 2009 (K. Albizati) 2 Mechanism This step is not favorable due to the great difference in pK a between ethanol (about 17) and ethyl acetate (about 25). Very little of the ester enolate is present at equilibrium , but it is enough for the reaction to proceed. This step is a simple nucleophilic addition of an enolate to a carbonyl. This step is an elimination step. Although alkoxide is a relatively strong base and is not a good leaving group in an S N 2 (displacement) sense, it is an OK leaving group in an elimination reaction under strongly basic conditions. ANIMATION MECHANISM Up to here, all of the reactions have been reversible; that is, the equilibrium constants are not all that favorable. However the final step in the mechanism is a strongly favored acid-base reaction and serves to drive the whole sequence to this thermodynamic endpoint:
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Chemistry 140C Spring 2009 (K. Albizati) 3 At this point, the reaction is quenched with aqueous acid and the -ketoester enolate is protonated and can be isolated and purified: In general, protons that are between 2 carbonyl groups have enhanced acidity because of resonance delocalization in the anion: And the hybrid can be drawn as : with the negative charge spread out over 5 atoms. See Table 23-1 for a list of pK a values of -dicarbonyls and related compounds. This is not for memorization but you should have an idea of the general neighborhood of the pKa of a -dicarbonyl and what is more or less acidic than these. The Claisen condensation fails for esters that have only one acidic hydrogen next to the ester carbonyl. Why do you suppose this is the case?
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Ch 23 notes - Chemistry 140C Spring 2009 (K. Albizati)...

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